Hybrid multi-channel/cue coding/decoding of audio signals

ABSTRACT

Part of the spectrum of two or more input signals is encoded using conventional coding techniques, while encoding the rest of the spectrum using binaural cue coding (BCC). In BCC coding, spectral components of the input signals are downmixed and BCC parameters (e.g., inter-channel level and/or time differences) are generated. In a stereo implementation, after converting the left and right channels to the frequency domain, pairs of left- and right-channel spectral components are downmixed to mono. The mono components are then converted back to the time domain, along with those left- and right-channel spectral components that were not downmixed, to form hybrid stereo signals, which can then be encoded using conventional coding techniques. For playback, the encoded bitstream is decoded using conventional decoding techniques. BCC synthesis techniques may then apply the BCC parameters to synthesize an auditory scene based on the mono components as well as the unmixed stereo components.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of co-pending U.S. application Ser. No.10/936,464, filed on Sep. 8, 2004, which (1) claimed the benefit of thefiling date of U.S. provisional application No. 60/585,703, filed onJul. 6, 2004, the teachings of which are incorporated herein byreference, and (2) was a continuation-in-part of the followingapplications, the teachings of all of which are incorporated herein byreference:

-   -   U.S. application Ser. No. 09/848,877 (“the '877 application”),        filed on May 4, 2001 and issued as U.S. Pat. No. 7,116,787 on        Oct. 3, 2006;    -   U.S. application Ser. No. 10/045,458 (“the '458 application”),        filed on Nov. 7, 2001, now abandoned, which itself claimed the        benefit of the filing date of U.S. provisional application No.        60/311,565, filed on Aug. 10, 2001;    -   U.S. application Ser. No. 10/155,437 (“the '437 application”),        filed on May 24, 2002 and issued as U.S. Pat. No. 7,006,636 on        Feb. 28, 2006;    -   U.S. application Ser. No. 10/246,570, filed on Sep. 18, 2002 and        issued as U.S. Pat. No. 7,292,901 on Nov. 6, 2007, which itself        claimed the benefit of the filing date of U.S. provisional        application No. 60/391,095, filed on Jun. 24, 2002; and    -   U.S. application Ser. No. 10/815,591, filed on Apr. 1, 2004,        which itself claimed the benefit of the filing date of U.S.        provisional application No. 60/544,287, filed on Feb. 12, 2004.

The subject matter of this application is related to the subject matterof U.S. patent application Ser. No. 10/246,165 (“the '165 application”),filed on Sep. 18, 2002 4 and issued as U.S. Pat. No. 7,039,204 on May 2,2006, the teachings of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the encoding of audio signals and thesubsequent decoding of the encoded audio signals to generate an auditoryscene during playback.

2. Description of the Related Art

In conventional stereo audio coding, the sum and the difference of theleft and right audio channels of the stereo input signal are formed andthen individually coded, e.g., using adaptive differential pulse codemodulation (ADPCM) or some other suitable audio coding algorithm, toform an encoded audio bitstream. The corresponding conventional stereoaudio decoding involves reversing the (ADPCM) coding algorithm torecover decoded sum and difference signals, from which left and rightaudio channels of a decoded stereo output signal are generated.

Although such conventional stereo audio coding/decoding (codec)techniques can produce an auditory scene during playback that accuratelyreflects the fidelity of the stereo input signal, the amount of datarequired for the corresponding encoded audio bitstream may beprohibitively large for some applications involving limited storagespace and/or transmission bandwidth.

SUMMARY OF THE INVENTION

The '877, '458, and '437 applications describe audio codec techniquesthat can produce smaller encoded audio bitstreams for the same orsubstantially similar levels of playback fidelity as those associatedwith conventional stereo audio codecs. In particular, these patentapplications are related to an audio coding technique referred to asbinaural cue coding (BCC).

When BCC coding is applied to stereo audio, the left and right channelsof the stereo input signal are downmixed (e.g., by summing) to a singlemono signal, which is then encoded using a suitable conventional audiocoding algorithm such as ADPCM. In addition, the left and right channelsare analyzed to generate a stream of BCC parameters. In oneimplementation, for each audio frame (e.g., 20 msec), the BCC parametersinclude auditory spatial cues such as an inter-channel or inter-aurallevel difference (ILD) value and an inter-channel or inter-aural timedifference (ITD) value between the left and right channels for each of aplurality of different frequency bands in the stereo input signal. Sincethe corresponding encoded audio data might include only an encoded monosignal and a stream of BCC parameters, the amount of encoded data may beconsiderably smaller (e.g., 50-80%) than that for a correspondingencoded audio bitstream generated using conventional stereo audiocoding, such as that described previously.

The corresponding BCC decoding involves reversing the (e.g., ADPCM)coding algorithm to recover a decoded mono signal. Stereo audiosynthesis techniques are then applied to the decoded mono signal usingthe BCC parameters to generate left and right channels of a decodedstereo audio signal for playback. Although typically lower than thatachieved using conventional stereo audio codecs, the fidelity of anauditory scene generated using BCC coding and decoding may be acceptablefor many applications, while typically using lower bandwidth.

Embodiments of the present invention are related to a hybrid audio codectechnique in which conventional audio coding is applied to certainfrequency bands of the input audio signals, while BCC coding is appliedto other frequency bands of the input audio signals. In one possiblestereo implementation, signal spectral components whose frequenciesabove a specified threshold frequency (e.g., 1.5 kHz) are coded usingBCC coding, while lower-frequency components are coded usingconventional stereo coding. As a result, even higher fidelity playbackcan be achieved than using only BCC coding, while still reducing thetotal amount of encoded data compared to conventional stereo coding.

According to one embodiment, the present invention is a method forencoding N input audio signals, where N>1. Each of the N input audiosignals is converted into a plurality of spectral components in afrequency domain. For each of one or more, but not all, of the spectralcomponents, the spectral components corresponding to the N input audiosignals are downmixed to generate a downmixed spectral component,leaving one or more of the spectral components for each of the N inputaudio signals unmixed. An encoded audio bitstream is generated based onthe one or more downmixed spectral components and one or more unmixedspectral components.

According to another embodiment, the present invention is an encodedaudio bitstream generated by performing the previously recited method.

According to another embodiment, the present invention is an apparatusfor processing N input audio signals, where N>1, for encoding. One ormore transforms are configured to convert each of the N input audiosignals into a plurality of spectral components in a frequency domain. Adownmixer is configured, for each of one or more, but not all, of thespectral components, to downmix the spectral components corresponding tothe N input audio signals to generate a downmixed spectral component,leaving one or more of the spectral components for each of the N inputaudio signals unmixed.

According to another embodiment, the present invention is a method fordecoding an encoded audio bitstream. The encoded audio bitstream isdecoded to generate a plurality of spectral components in a frequencydomain, wherein one or more sets of the spectral components correspondto downmixed spectral components, and one or more sets of the spectralcomponents correspond to unmixed spectral components. For each set ofthe downmixed spectral components, one or more auditory spatialparameters are applied to generate a synthesized spectral component. Thesynthesized spectral components and the unmixed spectral components areconverted into N decoded audio signals in a time domain, where N>1.

According to another embodiment, the present invention is an apparatusfor decoding an encoded audio bitstream. An audio decoder is configuredto decode the encoded audio bitstream to generate a plurality ofspectral components in a frequency domain, wherein one or more sets ofthe spectral components correspond to downmixed spectral components, andone or more sets of the spectral components correspond to unmixedspectral components. A synthesizer is configured, for each set of thedownmixed spectral components, to apply one or more auditory spatialparameters to generate a synthesized spectral component. One or moreinverse transforms are configured to convert the synthesized spectralcomponents and the unmixed spectral components into N decoded audiosignals in a time domain, where N>1.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and advantages of the present invention willbecome more fully apparent from the following detailed description, theappended claims, and the accompanying drawings in which:

FIG. 1 shows a block diagram of a hybrid audio system, according to oneembodiment of the present invention;

FIG. 2 shows a block diagram of the processing implemented by the BCCanalyzer/mixer of FIG. 1, according to one embodiment of the presentinvention; and

FIG. 3 shows a block diagram of the processing implemented by the BCCsynthesizer of FIG. 1, according to one embodiment of the presentinvention.

DETAILED DESCRIPTION

FIG. 1 shows a block diagram of a hybrid audio system 100, according toone embodiment of the present invention. Audio system 100 comprisestransmitter 102 and receiver 104. Transmitter 102 receives the left (L)and right (R) channels of an input stereo audio signal and generates anencoded audio bitstream 106 and a corresponding stream 108 of BCCparameters, which, depending on the implementation, may or may not beexplicitly encoded into bitstream 106. FIG. 1 shows BCC parameter stream108 being transmitted out-of-band from transmitter 102 to receiver 104.In either case, receiver 104 receives the data generated by transmitter102, decodes encoded audio bitstream 106, and applies the BCC parametersin stream 108 to generate the left (L′) and right (R′) channels of adecoded stereo audio signal.

More particularly, transmitter 102 comprises BCC analyzer/mixer 110 andstereo audio coder 112, while receiver 104 comprises stereo audiodecoder 114 and BCC synthesizer 116.

In transmitter 102, BCC analyzer/mixer 110 converts the left (L) andright (R) audio signals into the frequency domain. For spectralcomponents above a specified threshold frequency, BCC analyzer/mixer 110generates BCC parameters for stream 108 using the BCC techniquesdescribed in the '877, '458, and '437 applications. BCC analyzer/mixer110 also downmixes those high-frequency components to mono. Copies ofthe high-frequency mono component are then converted back to the timedomain in combination with the low-frequency “unmixed” left and rightcomponents (i.e., the unprocessed frequency-domain components below thespecified threshold frequency), respectively, to form hybrid left andright signals 118. Stereo audio coder 112 applies conventional stereocoding to these hybrid left and right signals to generate encoded audiobitstream 106.

FIG. 2 shows a block diagram of the processing implemented by BCCanalyzer/mixer 110 of FIG. 1, according to one embodiment of the presentinvention. Fast Fourier transform (FFT) 202 _(L) converts the left audiosignal L into a plurality of left-channel spectral components 204 in thefrequency domain. Similarly, FFT 202 _(R) converts the right audiosignal R into a plurality of right-channel spectral components 206 inthe frequency domain. The one or more left-channel components 204 _(HI)and the corresponding one or more right-channel components 206 _(HI)whose frequencies are above the specified threshold frequency areapplied to both downmixer 208 and BCC parameter generator 216.

Downmixer 208 combines each high-frequency left-channel component 204_(HI) with its corresponding high-frequency right-channel component 206_(HI) to form a high-frequency mono component 210 _(HI). The processingperformed by downmixer 208 to generate the mono components may vary fromimplementation to implementation. In one possible implementation,downmixer 208 simply averages the corresponding left- and right-channelcomponents. In another possible implementation, downmixer 208 implementsthe downmixing technique described in the '165 application. Thoseskilled in the art will appreciate that other suitable downmixingalgorithms are possible.

Replicator 212 generates two copies of each high-frequency monocomponent 210 _(HI) for application to left and right inverse FFTs(IFFTs) 214 _(L) and 214 _(R), respectively. IFFTs 214 _(L) and 214 _(R)also receive the low-frequency left and right components 204 _(LO) and206 _(LO), respectively, from FFTs 202 _(L) and 202 _(R). IFFTs 214 _(L)and 214 _(R) convert their respective sets of components back to thetime domain to generate the left and right hybrid signals 118 _(L) and118 _(R), respectively. The resulting two-channel signal containsidentical frequency components within spectral regions that wereconverted to mono, with the remaining parts being identical to the inputsignals L and R. As a result, stereo audio coder 112 will typicallygenerate an encoded audio bitstream that has fewer bits than if it wereto encode the original input stereo audio signal (L and R).

BCC parameter generator 216 analyzes the high-frequency left and rightcomponents 204 _(HI) and 206 _(HI) to generate BCC parameters for stream108 of FIG. 1 for each frequency band above the specified thresholdfrequency.

Referring again to FIG. 1, in receiver 104, stereo audio decoder 114applies a conventional stereo decoding algorithm (e.g., to reverse thecoding implemented by coder 112) to recover hybrid decoded left andright signals 120. BCC synthesizer 116 applies BCC synthesis techniquesto the high-frequency portions of channels 120 to synthesize thehigh-frequency portions of the decoded left (L′) and right (R′)channels. In particular, BCC synthesizer 116 converts the hybridchannels 120 to the frequency domain, applies the BCC parameters to thehigh-frequency components to synthesize high-frequency left and rightcomponents using the BCC techniques described in the '877, '458, and'437 applications, and then reconverts the resulting synthesizedhigh-frequency components and corresponding decoded low-frequencycomponents to the time domain.

FIG. 3 shows a block diagram of the processing implemented by BCCsynthesizer 116 of FIG. 1, according to one embodiment of the presentinvention. FFT 302 _(L) converts hybrid left audio signal 120 _(L) fromstereo audio decoder 114 into a plurality of left-channel spectralcomponents 304 in the frequency domain. Similarly, FFT 302 _(R) convertshybrid right audio signal 120 _(R) from decoder 114 into a plurality ofright-channel spectral components 306 in the frequency domain. The oneor more left-channel components 304 _(HI) and the corresponding one ormore right-channel components 306 _(HI) whose frequencies are above thespecified threshold frequency are applied to mono signal generator 308.

Mono signal generator 308 generates a high-frequency mono component foreach high-frequency left-channel component 304 _(HI) and itscorresponding high-frequency right-channel component 306 _(HI). Ideally,since replicator 212 of FIG. 2 generated identical copies of eachhigh-frequency mono component 210 _(HI), each high-frequencyleft-channel component 304 _(HI) should be identical to itscorresponding high-frequency right-channel component 306 _(HI). As such,mono signal generator 308 could simply select either the left channel orthe right channel to “generate” the one or more high-frequency monocomponents 310 _(HI). Alternatively, mono signal generator 308 couldsimply average or perform some other suitable downmixing algorithm,including the algorithm described in the '165 application, to generateeach mono component 310 _(HI), in order to account for any real-worlddifferences that may exist between the left and right high-frequencycomponent 304 _(HI) and 306 _(HI).

In any case, BCC stereo synthesizer 312 applies BCC processing togenerate a high-frequency left-channel component 314 _(HI) and ahigh-frequency right-channel component 316 _(HI) for each high-frequencymono component 310 _(HI). The high-frequency left- and right-channelcomponents 314 _(HI) and 316 _(HI) are applied to left and right IFFTs318 _(L) and 318 _(R), respectively. IFFTs 214 _(L) and 214 _(R) alsoreceive the low-frequency left and right components 304 _(LO) and 306_(LO) respectively, from FFTs 302 _(L) and 302 _(R). IFFTs 318 _(L) and318 _(R) convert their respective sets of components back to the timedomain to generate left and right channels L′ and R′, respectively, ofthe decoded stereo signal of FIG. 1.

A natural cross-over frequency from the “true” stereo part to theBCC-generated stereo part is 1.5 kHz. Above that frequency the humanauditory system does not substantially evaluate inter-aural phasedifferences for sound localization. Thus, the human auditory system isless sensitive to inter-channel phase errors introduced by BCCprocessing in that range. Moreover, the most salient auditorylocalization cues are usually derived from low-frequency components,unless the audio signal has dominant spectral energy at higherfrequencies.

The present invention can also be implemented using a hybrid transmittersuch as transmitter 102 of FIG. 1, but a receiver that does not performany BCC processing. In this case, BCC synthesizer 116 of FIG. 1 may beomitted from receiver 104, and the resulting receiver can ignore BCCparameter stream 108 during decoding processing. Legacy receivers thatcontain only a conventional audio decoder fall into that category. Sucha receiver would not provide BCC spatialization of the auditory imagefor spectral parts of the decoded audio signals that are based on monocomponents. However, there is still a remaining stereo effect created bythose parts of the spectrum that are preserved as stereo. This stereoeffect by itself provides a mechanism for bit-rate reduction as comparedto the transmission of the full-bandwidth stereo. Explicitly, mixingparts of the spectrum of the audio input signal to mono reduces the bitrate of a conventional audio coder. The spatial image degradation shouldbe tolerable, if the mono part of the spectrum is limited to frequenciesabove about 1 kHz.

For some applications, BCC processing may be intentionally limited totransmit only inter-channel level differences as the BCC parameters(i.e., and not any inter-channel time differences). For headphoneplayback, inter-channel time differences are important for creating anatural spatial image, especially at frequencies below 1.5 kHz. Bykeeping the stereo signal up to a limit of about 1.5 kHz, the spatialcues in that frequency are available at the receiver and greatly improvethe listening experience with headphones.

Transmitting a small spectral bandwidth as a stereo signal does notnecessarily increase the bit rate of the audio coder dramaticallycompared to applying BCC processing to the full spectral range. Theaudio coder can still take full advantage of those parts of the spectrumthat are mono by using, e.g., sum/difference coding. The data rate forthe BCC parameters can be reduced, since no parameters need to betransmitted for the spectral part that is kept stereo.

The application of BCC processing to spectral regions can be madeadaptive such that an optimum quality/bit-rate tradeoff is achieved. Forinstance, BCC processing could be switched off for very criticalmaterial, or it could be applied to the full spectrum for non-criticalmaterial. The spectral region where BCC processing is applied can becontrolled, for instance, by one parameter per frame that indicates theupper frequency bound up to which the stereo signal is kept forencoding. In addition, the threshold frequency between stereo and BCCcoding could dynamically change based on the number of bits that wouldactually be used to code different spectral regions of the audio data bythe different techniques.

The audio quality range covered by the hybrid codec scheme in FIG. 1reaches transparent quality when the spectral region of BCC processinghas zero bandwidth. With continuously increasing bandwidth for BCCprocessing, a gradual quality transition from traditional stereo audiocoding to the original full-bandwidth BCC coding scheme of the '877,'458, and '437 applications is possible. Therefore, the quality range ofthe present invention extends to both quality ranges: that of theoriginal BCC scheme and that of the traditional audio coding scheme.

Moreover, the hybrid coding scheme is inherently bit-rate scalable. Interms of the coder structure, such a scheme is also referred to as“layered coding.” This feature can be used for instance to reduce thebit rate of a given bitstream to accommodate for channels with lowercapacity. For such purposes, the BCC parameters can be removed from thebitstream. In that case, a receiver is still able to decode an audiosignal with a reduced stereo image, as described above for the legacydecoder. A further step for reducing the bit rate is meaningful, if thestereo audio coder uses sum/difference coding. It is possible to isolatethe difference signal information in the bitstream and remove it. Inthis case, the receiver will decode only the sum signal, which is amonophonic audio signal.

The different “layers” (e.g., sum, difference, and BCC information) alsoprovide a natural division of the bitstream for unequal error protectionfor lossy channels. For such applications, the sum signal would get thehighest protection and the BCC information would get the lowestprotection. If the channel temporarily has a high error rate, then themono sum signal might still be recoverable, while the difference signaland BCC information might be lost. Such a scheme avoids more audiblyannoying frame concealment mechanisms.

Although the present invention has been described in the context ofapplications in which BCC processing is applied to all and onlyfrequency bands above a specified threshold frequency, the presentinvention is not so limited. In general, for the hybrid processing ofthe present invention, BCC processing can be applied to any one ormore—but less than all—frequency bands, whether they are contiguous ornot, and independent of any threshold frequency.

For example, in one possible implementation, BCC processing is appliedto only those frequency bands with energy levels below a specifiedthreshold energy, while conventional stereo encoding is applied to theremaining frequency bands. In this way, conventional stereo encodingoptimizes fidelity for the “important” (i.e., high spectral energy)frequency bands, while BCC processing optimizes bandwidth for theless-important (i.e., low spectral energy) frequency bands.

Although the present invention has been described in the context ofencoding and decoding a stereo audio signal, the present invention canalso be applied to multi-channel applications having more than two inputand output channels. Furthermore, the present invention can be appliedto applications in which the number of input channels differs from(either higher or lower than) the number of output channels.

Although the present invention has been described in the context ofreceivers that apply the BCC techniques of the '877, '458, and '437applications to synthesize auditory scenes, the present invention canalso be implemented in the context of receivers that apply othertechniques for synthesizing auditory scenes that do not necessarily relyon the techniques of the '877, '458, and '437 applications.

Although the present invention has been described in the context of areal-time system in which the generated data are transmitted immediatelyfrom the transmitter to the receiver for real-time decoding andplayback, the invention is not so limited. For example, the datagenerated by the transmitter may be stored in computer memory or otherelectronic storage medium for subsequent, non-real-time playback by oneor more receivers.

Although the present invention has been described in the context ofembodiments having an audio coder (e.g., stereo coder 112 of FIG. 1)that encodes hybrid signals in the time domain to generate an encodedaudio bitstream and an audio decoder (e.g., stereo decoder 114) thatdecodes the encoded audio bitstream to recover decoded hybrid signals inthe time domain, the present invention is not so limited. Those skilledin the art will understand that the present invention can be implementedin the context of embodiments that code and decode audio data in thefrequency domain. For example, the embodiment of FIGS. 1-3 can bemodified to replace stereo audio coder 112 and stereo audio decoder 114with audio codecs that encode and decode, respectively, audio data inthe frequency domain. In that case, BCC analyzer/mixer 110 of FIG. 2 canbe modified to eliminate replicator 212 and IFFTs 214, and BCCsynthesizer 116 of FIG. 3 can be modified to eliminate FFTs 302 and monosignal generator 308. In that case, downmixed (i.e., mono) spectralcomponents 210 _(HI) generated by downmixer 208 and unmixed spectralcomponents 204 _(LO) and 206 _(LO) are passed directly to thefrequency-domain audio coder in the transmitter. Similarly, thecorresponding downmixed (i.e., mono) and unmixed spectral componentsrecovered by the frequency-domain audio decoder in the receiver arepassed directly to BCC stereo synthesizer 312 and IFFTs 318,respectively.

The present invention may be implemented as circuit-based processes,including possible implementation on a single integrated circuit. Aswould be apparent to one skilled in the art, various functions ofcircuit elements may also be implemented as processing steps in asoftware program. Such software may be employed in, for example, adigital signal processor, micro-controller, or general-purpose computer.

The present invention can be embodied in the form of methods andapparatuses for practicing those methods. The present invention can alsobe embodied in the form of program code embodied in tangible media, suchas floppy diskettes, CD-ROMs, hard drives, or any other machine-readablestorage medium, wherein, when the program code is loaded into andexecuted by a machine, such as a computer, the machine becomes anapparatus for practicing the invention. The present invention can alsobe embodied in the form of program code, for example, whether stored ina storage medium, loaded into and/or executed by a machine, ortransmitted over some transmission medium or carrier, such as overelectrical wiring or cabling, through fiber optics, or viaelectromagnetic radiation, wherein, when the program code is loaded intoand executed by a machine, such as a computer, the machine becomes anapparatus for practicing the invention. When implemented on ageneral-purpose processor, the program code segments combine with theprocessor to provide a unique device that operates analogously tospecific logic circuits.

It will be further understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated in order to explain the nature of this invention may be madeby those skilled in the art without departing from the scope of theinvention as expressed in the following claims.

1. A transmitter-implemented method for encoding N input audio signals,N>1, comprising the steps of: (a) the transmitter converting each of theN input audio signals into a plurality of spectral components in afrequency domain; (b) for each of one or more, but not all, of thespectral components, the transmitter downmixing the spectral componentscorresponding to the N input audio signals to generate a downmixedspectral component, leaving one or more of the spectral components foreach of the N input audio signals unmixed; and (c) the transmittergenerating an encoded audio bitstream based on the one or more downmixedspectral components and one or more unmixed spectral components, whereinstep (c) comprises the steps of: (1) the transmitter replicating eachdownmixed spectral component to generate a plurality of replicateddownmixed spectral components; (2) the transmitter converting theplurality of replicated downmixed spectral components and the one ormore unmixed spectral components into N hybrid audio signals in a timedomain; and (3) the transmitter applying an audio coding algorithm tothe N hybrid audio signals to generate the encoded audio bitstream. 2.The method as claimed in claim 1, wherein step (b) further comprises thestep of generating one or more auditory spatial parameters for the oneor more downmixed spectral components.
 3. The method as claimed in claim1, wherein: N=2; the two input audio signals correspond to left andright input audio signals of a stereo input audio signal; each downmixedspectral component is a mono spectral component; and the encoded audiobitstream is generated using a stereo audio coder.
 4. The method asclaimed in claim 1, wherein: the one or more downmixed spectralcomponents have frequencies above a specified threshold frequency; andthe one or more unmixed spectral components have frequencies below thespecified threshold frequency.
 5. The method as claimed in claim 4,wherein the specified threshold frequency varies dynamically over time.6. The method as claimed in claim 4, wherein the specified thresholdfrequency varies as a function of bit rate.
 7. The method as claimed inclaim 1, wherein: the one or more downmixed spectral components havespectral energies below a specified threshold energy; and the one ormore unmixed spectral components have spectral energies above thespecified threshold energy.
 8. An apparatus for processing N input audiosignals, N>1 for encoding, comprising: (a) one or more transformsconfigured to convert each of the N input audio signals into a pluralityof spectral components in a frequency domain; and (b) a downmixerconfigured, for each of one or more, but not all, of the spectralcomponents, to downmix the spectral components corresponding to the Ninput audio signals to generate a downmixed spectral component, leavingone or more of the spectral components for each of the N input audiosignals unmixed, wherein the apparatus is configured to replicate eachdownmixed spectral component to generate a plurality of replicateddownmixed spectral components and convert the plurality of replicateddownmixed spectral components and the one or more unmixed spectralcomponents into N hybrid audio signals in a time domain.
 9. Theapparatus of claim 8, further comprising an audio coder configured toapply an audio coding algorithm to the N hybrid audio signals togenerate an encoded audio bitstream.
 10. A receiver-implemented methodfor decoding an encoded audio bitstream, the method comprising the stepsof: (a) the receiver decoding the encoded audio bitstream to generate aplurality of spectral components in a frequency domain, wherein: one ormore sets of the spectral components correspond to replicated downmixedspectral components; and one or more sets of the spectral componentscorrespond to unmixed spectral components; (b) for each set of thereplicated downmixed spectral components, the receiver converting thereplicated downmixed spectral components into a single downmixedspectral component and applying one or more auditory spatial parametersto the single downmixed spectral component to generate a plurality ofsynthesized spectral components; and (c) the receiver converting thesynthesized spectral components and the unmixed spectral components intoN decoded audio signals in a time domain, N>1, wherein step (a)comprises the steps of: (1) the receiver decoding the encoded audiobitstream to recover N hybrid audio signals; and (2) the receiverconverting each of the N hybrid audio signals into the plurality ofspectral components in the frequency domain.
 11. The method as claimedin claim 10, wherein: N=2; the encoded audio bitstream is decoded usinga stereo audio decoder; the two hybrid audio signals correspond to leftand right hybrid audio signals of a hybrid stereo audio signal; and eachdownmixed spectral component is a mono spectral component.
 12. Themethod as claimed in claim 10, wherein: the one or more downmixedspectral components have frequencies above a specified thresholdfrequency; and the one or more unmixed spectral components havefrequencies below the specified threshold frequency.
 13. The method asclaimed in claim 12, wherein the specified threshold frequency variesdynamically over time.
 14. The method as claimed in claim 12, whereinthe specified threshold frequency varies as a function of bit rate. 15.The method as claimed in claim 10, wherein: the one or more downmixedspectral components have spectral energies below a specified thresholdenergy; and the one or more unmixed spectral components have spectralenergies above the specified threshold energy.
 16. An apparatus fordecoding an encoded audio bitstream, the apparatus comprising: (a) anaudio decoder configured to decode the encoded audio bitstream togenerate a plurality of spectral components in a frequency domain,wherein: one or more sets of the spectral components correspond toreplicated downmixed spectral components; and one or more sets of thespectral components correspond to unmixed spectral components; (b)asynthesizer configured, for each set of the replicated downmixedspectral components, to convert the replicated downmixed spectralcomponents into a single downmixed spectral component and apply one ormore auditory spatial parameters to the single downmixed spectralcomponent to generate a plurality of synthesized spectral components;and (c) one or more inverse transforms configured to convert thesynthesized spectral components and the unmixed spectral components intoN decoded audio signals in a time domain, N>1, wherein the audio decoderis configured to: (1)decode the encoded audio bitstream to recover Nhybrid audio signals; and (2)convert each of the N hybrid audio signalsinto the plurality of spectral components in the frequency domain. 17.The apparatus as claimed in claim 16, wherein: N=2; the audio decoder isa stereo audio decoder; the two hybrid audio signals correspond to leftand right hybrid audio signals of a hybrid stereo audio signal; and eachdownmixed spectral component is a mono spectral component.